Refactor cohort methods into service

This is step 2 of implementing cohorts whose memberships are entire
wiki projects.  In the next step we'll implement logical cohort types
that sit on top of the CohortStore database table mapping.  These will
need to be coordinated by a service, and so this change is implementing
that service.

Change-Id: I7a31964cf7af8c1c1a7d1b129a7a62cd0eb7c691